With outstanding advancement of electronics technology in recent years, portable electronic devices have been made smaller, lighter, and thinner, and equipped with multiple functions. According to this, batteries as power sources for electronic devices are required to be smaller, lighter, thinner, and highly reliable. In response to the demand, a generally-proposed multilayer lithium ion secondary battery has a plurality of positive layers and a plurality of negative layers, which are laminated on each other via an electrolyte layer.
The multilayer lithium ion secondary battery is assembled by laminating battery cells with the thickness of several tens of μm. Therefore, the battery can be readily made smaller, lighter, and thinner. An all-solid lithium ion secondary battery includes solid electrolyte instead of electrolytic solution. Therefore, the all-solid lithium ion secondary battery is immune to leakage or depletion of liquid and has high reliability. Furthermore, because the all-solid lithium ion secondary battery includes lithium, the all-solid lithium ion secondary battery provides high voltage and high energy density.
As illustrated in FIG. 6, an all-solid lithium ion battery that includes an inorganic solid electrolyte for an electrolyte layer is produced by forming a laminated body by laminating a positive active material layer and a negative active material layer one by one with an electrolyte layer interposed therebetween.
For example, a negative unit is produced by printing a negative active material 101a, a negative current collector 102a, and a negative active material 103a onto a solid electrolyte 100a formed on a substrate made of PET for example. Similarly to the negative unit 104a, a positive unit 104b is produced by printing a positive active material 101b, a positive current collector 102b, and a positive active material 103b onto solid electrolyte 100b formed on a substrate made of PET for example.
At this time, in the negative unit 104a, a step 105a is provided at the left side of the drawing. On the other hand, in the positive unit 104b, a step 105b is provided at the right side of the drawing. By providing such steps, extraction from a current collector of the same pole can be done at once.
However, in the structure illustrated in FIG. 6, printing needs to be done three times only to produce the negative unit 104a. Besides, a base sheet is spoiled every time printing is done (called “sheet attack”). In addition, the step 105a has the thickness equivalent to the thickness of three layers of the active material 101a, the negative current collector 102a, and the active material 103a. The size of the step 105a has been a restriction on design or process.
With respect to the structure illustrated in FIG. 6, Patent Document 1 discloses a technology to further simplify the structure, and facilitate production.
That is, the technology disclosed in Patent Document 1 discloses an all-solid secondary battery having: a single active material layer made of a single layer including an active material; a positive current collecting electrode provided on one surface of the single active material layer; and a negative current collecting electrode provided on the other surface of the single active material layer. Here, the active material is capable of a plurality of valence fluctuations, and has different redox potentials, which correspond to each valence fluctuation.
This technology is a technology employing a single active material layer formed by a single layer including an active material that can be either a positive electrode or a negative electrode, instead of an inner electrode having a three-layer structure (positive active material layer, electrolyte layer, and negative active material layer) as illustrated in FIG. 6. Patent Document 1 also discloses a case where solid electrolyte is further included in a single active material layer.
The technology disclosed in Patent Document 1 defines three layers of a positive current collecting electrode, a negative current collecting electrode, and a single active material layer as one unit. Therefore, its structure is simple and production is easy.
However, once a secondary battery was implemented by using this technology, it was found that a short circuit occurs.